Spin-polarized surface states on Fe-deposited Au(111) surface: A theoretical study

We have studied electronic structure of Fe-deposited Au(111) by performing ab initio density functional theory calculations. We find that the magnetic moment on the deposited Fe layer is enhanced as compared to that in bulk iron. We observe a large number of new states on the Fe-deposited surface — one of which is in the majority spin channel having similar dispersion to that on the clean surface, and others in the minority spin channel. The effective mass of electrons in surface states near the Fermi level increases on Fe deposition. The electronic properties are found to be insensitive to the stacking of near-surface layers. We need to use very thick slabs in our calculations to avoid splitting of surface states due to spurious interactions between the two surfaces of the slab. Using the local density of states profiles for different surface states, we conclude that in scanning tunneling microscope experiments one can detect two of the surface states — one in the majority channel below the Fermi level, and another in the minority channel appearing just above the Fermi energy. We compare our results to those from scanning tunneling spectroscopy experiments.     

Valence surface electronic states on Ge(001)

The optical, electrical, and chemical properties of semiconductor surfaces are largely determined by their electronic states close to the Fermi level (E{F}). We use scanning tunneling microscopy and density functional theory to clarify the fundamental nature of the ground state Ge(001) electronic structure near E{F}, and resolve previously contradictory photoemission and tunneling spectroscopy data. The highest energy occupied surface states were found to be exclusively back bond states, in contrast to the Si(001) surface, where dangling bond states also lie at the top of the valence band.     

Electronic states in magnetic nanostructures

The electronic states responsible for oscillatory magnetic coupling, giant magnetoresistance (GMR), and spin-polarized tunneling are explored. They occupy well-defined locations in (E, k) space. Their energy E has to be within a few kT of the Fermi level, a range that is now becoming accessible to high-resolution photoemission. Particular attention is paid to k-regions near the Fermi level crossings of the s, p-band, where a sizable group velocity is combined with a d-like magnetic splitting and spin-polarization. These electronic states can be tailored by quantization in structures with single-digit nanometer dimensions, such as two-dimensional quantum wells and one-dimensional arrays of stripes and dots. Such arrays can be produced by self-assembly on top of stepped silicon surfaces.     

A new aspect of superconductivity in A-15 compounds

We present a new aspect of superconductivity in A-15 compounds which is able to explain their exceptional role among the highT _c superconductors. The basic idea is that a strong energy dependence of the electronic density of states near the Fermi level may greatly reduce the repulsive part of the frequency dependent electron-phonon interaction. This leads to a large enhancement ofT _c which is a maximum when the Fermi energy is comparable to a typical phonon energy. Our findings are based on numerical solutions of the Eliashberg equations where both the retardation of the electron-phonon coupling and the energy dependence of the electronic density of states have been included. For the electronic density of states we use the models of Labbé und Friedel and of Cohen et al., while the shape of the Eliashberg function ² F() is taken from the tunneling results of Shen. We compare our theory to experimental results for ternary A-15 compounds.     

Local electronic structures in electron-doped cuprates with coexisting orders

Motivated by the observation of a so-called non-monotonic gap in recent angle-resolved photoemission spectroscopy measurement, we study the local electronic structure near impurities in electron-doped cuprates by considering the influence of antiferromagnetic (AF) spin-density-wave (SDW) order. We find that the evolution of density of states (DOS) with AF SDW order clearly indicates the non-monotonic d-wave gap behavior. More interestingly, the local DOS for spin-up is much different from that for spin-down with increasing AF SDW order. As a result, the impurity induced resonance state near the Fermi energy exhibits a spin-polarized feature. These features can be detected by spin-polarized scanning tunneling microscopy experiments.     

A spin-selective approach for surface states at Co nanoislands

During recent years the surface electronic states of cobalt nanoislands grown on Cu(111) and Au(111) have been extensively studied and still yield fascinating results. Among magnetic surfaces, cobalt islands are particularly appealing because of their spin-polarized electronic states near the Fermi energy, involving localized d states of minority character, as well as free-like s–p states of majority character. We show here that these states are a sensitive probe to minute changes of structural details such as strain and stacking, and therefore constitute an ideal playground to study the interplay between structural and spin-related properties. Due to their size, cobalt islands on Cu(111) offer the additional opportunity to host single-magnetic adsorbates suitable for spin-polarized scanning tunneling microscopy and spectroscopy (SP-STM and SP-STS). We establish here that, in an energy interval just below the Fermi level, the spin-polarization of a transition-metal atom is governed by surface-induced states opposite in sign compared to the island, while the spin-polarization of Co-Phthalocyanine molecules is governed by molecular states. This opens up interesting perspectives for controlling and engineering spin-polarized phenomena at the nanoscale.     

硼烯纳米带能带结构和态密度的第一性原理研究

基于密度泛函理论,采用第一性原理的方法计算H修饰边缘不同宽度硼稀纳米带的电荷密度、电子能带结构、总态密度和分波态密度。结果表明,硼烯纳米带的宽度大小影响着材料的导电性能,宽度5的硼烯纳米带是间接带隙简并半导体,带隙值为0.674 eV,而宽度7的硼烯纳米带却具有金属材料的性质。分波态密度表明,宽度5的硼烯纳米带的费米能级附近主要是由B-2s、2p电子态贡献,H-1s主要贡献于下价带且具有局域性,消除了材料边缘的不稳定性。宽度7的B-2p和H-1s电子态贡献的导带和价带处于主导地位,费米能级附近B-2p和H-1s电子态的杂化效应影响材料的整体发光性能。     

Detection of Fe 3d electronic states in valence band and magnetic properties of Fe-doped ZnO film

Fe-doped ZnO film has been grown by laser molecular beam epitaxy (L-MBE) and structurally characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM), all of which reveal the high quality of the film. No secondary phase was detected. Resonant photoemission spectroscopy (RPES) with photon energies around the Fe 2p-3d absorption edge is performed to detect the electronic structure in the valence band. A strong resonant effect at a photon energy of 710 eV is observed. Fe³⁺ is the only valence state of Fe ions in the film and the Fe 3d electronic states are concentrated at binding energies of about 3.8 eV and 7 eV～ 8 eV. There are no electronic states related to Fe near the Fermi level. Magnetic measurements reveal a typical superparamagnetic property at room temperature. The absence of electronic states related to Fe near the Fermi level and the high quality of the film, with few defects, provide little support to ferromagnetism.     

非晶碳结构建模和电子结构的第一性原理研究

基于密度泛函理论，采用了一种更为精确的交换相关泛函OLYP(OPTX+LYP)，对密度范围从2.0到3.2 g/cm³的非晶碳进行结构建模. 模拟得到的5个碳网络结构无论从径向分布函数还是sp³含量都与实验符合得很好. 对非晶碳电子结构的研究表明费米能级附近的电子态密度主要是sp²碳原子的贡献. 随着密度的增加，sp³碳原子增加，费米能级附近的态密度越来越小. 小环结构增加了费米能级附近的电子态密度，缺陷态在费米能级形     

非晶碳结构建模和电子结构的第一性原理研究

基于密度泛函理论,采用了一种更为精确的交换相关泛函OLYP(OPTX+LYP),对密度范围从2.0到3.2 g/cm³的非晶碳进行结构建模. 模拟得到的5个碳网络结构无论从径向分布函数还是sp³含量都与实验符合得很好. 对非晶碳电子结构的研究表明费米能级附近的电子态密度主要是sp²碳原子的贡献. 随着密度的增加,sp³碳原子增加,费米能级附近的态密度越来越小. 小环结构增加了费米能级附近的电子态密度,缺陷态在费米能级形 关键词： 非晶碳 密度泛函理论 电子结构     

Surface-state depopulation on small Ag(111) terraces

The dependence of the local density of states near the Fermi energy E(F) on the width of terraces T is investigated by tunneling scanning spectroscopy on Ag(111) at 7 K. With decreasing T, the electronic density in the occupied surface state shifts monotonically towards E(F), leading to a depopulation at T=3.2 nm in quantitative agreement with a Fabry-Pérot model. Depopulation coincides with a switch from confinement by terrace modulation to step modulation.     

Quantum confinement between self-organized Pt nanowires on Ge(001)

The existence of one-dimensional (1D) electronic states between self-organized Pt nanowires spaced 1.6 or 2.4 nm apart on a Ge(001) surface is revealed by low-temperature scanning tunneling microscopy. These perfectly straight Pt nanowires act as barriers for a surface state (located just below the Fermi level) of the underlying terrace. The energy positions of the 1D electronic states are in good agreement with the energy levels of a quantum particle in a well. Spatial maps of the differential conductivity of the 1D electronic states conclusively reveal that these states are exclusively present in the troughs between the Pt nanowires.     

Hydrogen adsorption on Gd(0001)

The electronic structure of hydrogen adsorbate-induced states on Gd(0001) was investigated by means of photoelectron spectroscopy with linearly polarized radiation. The E vector of the incoming photon beam is rotatable. Clean and well-ordered rare-earth (0001) surfaces exhibit a highly localized surface state near the Fermi edge. After the adsorption of hydrogen, the surface state disappears and an additional sharp feature at about 4 eV binding energy is observed. For this latter state, the ratio of the radial matrix elements as well as the relative phase shifts were determined to be R=R_p/R_f=2.4±0.3 and δ_f−δ_p=310±10°, respectively. The removal of the Gd surface state by hydrogen adsorption was investigated by means of scanning tunneling microscopy (STM) and spectroscopy (STS). The removal of the surface state exhibits domain-like behavior, with surface steps acting as domain boundaries. The tunneling spectra reveal that hydrogen adsorption causes a dramatic reduction in the differential conductivity near the Fermi level.     

SrRuO3的电子结构与磁性研究

用自洽的全势能线性丸盒轨道能带方法计算了氧化物体系SrRuO₃(SRO)的电子结构和磁性.对于理想的立方钙钛矿结构的计算得出的电子结构明显改善了已有的计算结果:每个元胞的磁矩为129μ_B,按原子球划分为084μ_B/Ru原子和011μ_B/O原子;Sr原子上的自旋磁矩几乎为零;费米能级处的态密度N(E_F)为435(states/Ryd/f.u.).关于实际的正交结构SRO,计算得出磁矩为108μ关键词： 过渡金属氧化物 电子结构 磁性     

钴原子及其团簇在Rh(111)和Pd(111)表面的扫描隧道显微学研究

利用扫描隧道显微镜和扫描隧道谱(STM/STS)及单原子操纵,系统研究了单个钴原子(Co) 及其团簇在Rh (111)和Pd (111)两种表面的吸附和自旋电子输运性质. 发现单个Co原子在Rh (111)上有两种不同的稳定吸附位,分别对应于hcp和fcc空位, 他们的高度明显不同,在针尖的操纵下单个Co原子可以在两种吸附位之间相互转化. 在这两种吸附位的单个Co原子的STS谱的费米面附近都存在很显著的峰形结构, 经分析认为Rh (111)表面单个Co原子处于混价区,因此这一峰结构是d轨道共振 和近藤共振共同作用的结果.对于Rh (111)表面上的Co原子二聚体和三聚体, 其费米面附近没有观测到显著的峰,这可能是由于原子间磁交换相互作用 和原子间轨道杂化引起的体系态密度改变所共同导致.与Rh (111)表面不同, 在Pd (111)表面吸附的单个Co原子则表现出均一的高度.并且对于Pd (111)表面所有 单个Co原子及其二聚体和三聚体,在其STS谱的费米面附近均未探测到显著的电子结构, 表明Co原子吸附于Pd (111)表面具有与Rh (111)表面上不同的原子-衬底相互作用与自旋电子输运性质.     

Tunneling electron induced bromine hopping on Si(100)-(2 x 1)

Tunneling electrons from the tip of a scanning tunneling microscope can be used to induce adatom hopping on Br-terminated Si(100)-(2x1) at low current and without voltage pulses. Hopping does not occur when electrons tunnel from a sample to a tip. The threshold energy is +0.8 V, and tunneling spectroscopy shows antibonding Si-Br states 0.8 eV above the Fermi level. Electron capture in these states is a necessary condition for hopping, but repulsive adsorbate interactions that lower the activation barrier are also required. Such interactions are strong near saturation for Br but are insufficient when the coverage is low or when Br is replaced by Cl.     

Ferromagnetism in transition-metal-doped II–VI compounds

We report systematical density-functional calculations of ternary transition-metal compounds based on zincblende ZnTe and CdTe semiconductor. Some of them are half-metallic at their optimized cell volumes. The effect of atomic position optimization (and cell volume re-optimization) on the electronic structures is to widen the energy bands near the Fermi energy and to reduce the density of states there. As a result, the Fermi level moves upward and the energy gap of the minority-spin bands at the Fermi energy becomes narrower. Therefore, the half-metallic gaps are reduced, two of them even being closed. These compounds are compatible with the II–VI semiconductors, and could be useful in spin-based electronics.     

自旋轨道耦合作用下石墨烯pn结的电子输运性质

本文研究了自旋轨道耦合作用下石墨烯纳米带pn结的电子输运性质. 当粒子的入射能量处于pn结两端势能之间时, 粒子将会以隧穿的形式通过石墨烯pn结, 同时伴随着电子空穴转换. 电导随费米能的变化曲线呈不等高阶梯状, 并在费米能位于pn结两端能量中点时取得最大值. 随着石墨烯pn结长度的增加, 电导以指数形式衰减. 自旋轨道耦合作用导致的能隙会使电导显著减小, 而边缘态的粒子则可以几乎毫无阻碍地通过pn结. 本文用一个简单的子带隧穿模型解释了上述特征. 最后还研究了在pn转换区中掺入替位杂质的情况. 在弱杂质下, 电导随费米能变化的曲线将不再对称; 当杂质较强时, 仅边界态的形成的电导台阶能够保持.     

Local spectroscopy of two-dimensional states in Na films on Cu(111)

Alkali overlayers on noble metal surfaces exhibit two-dimensional electronic states. We investigated the electronic structure of Na on Cu(111) at T=5 K with scanning tunneling spectroscopy. In agreement with photoelectron spectroscopy we found two electronic bands close to the Fermi energy which correspond to quantum well states (QWS). In second monolayer islands, confinement of the QWS was observed.